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Thread: High speed air bleeds

  1. #1

    High speed air bleeds

    I just got my motor back in my car and running but now it has a little stumble from about 3500-4200 rpm when you ease into the throttle. If I just whack the throttle hard it revs clean. It did not do this on the dyno but I had my high speed air bleeds on my Demon carb drilled out to supposedly the same size that they were on the dyno. I am thinking that they got drilled out to big. What do you guys think, is that the cause?

  2. #2
    Senior Member DYNO OPERATOR
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    The biggest advantage of using a dyno to tune is that you prove the parts you are going to race with. If you are not using the same carb that you dyno'ed with you know nothing about the mixture now and no doubt have possibly a huge fuel curve change by changing the bleeds after the dyno.
    Get the bleeds back and carb set up you dyno'ed with and then look for where the problem is if you still have it then. Ignition and other things could be at fault if you know that the carb is working well which you can not be sure of now.
    The demon carb is not the best choice for racing but if this is what you dyno'ed with successfully, then set it back where you were it and change step by step so you know what change the engine hates.
    Bruce

  3. #3
    Senior Member RACING JUNKIE
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    could be, i would try a size smaller bleed, and see if that fattened it enough, i would go in small steps, to maintain dyno numbers. jmo,

  4. #4
    Senior Member RACING JUNKIE
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    topspeed type faster than me, ( what he said).

  5. #5
    I think I confused topseed, everything is the same as it was on the dyno. We used the shops air bleeds while on the dyno and then I had mine drilled to supposedly the same size as the ones were on the dyno. The problem is that demon does not make air bleeds like a Holley or Quickfuel bleed, you can not buy them sized already, you have to buy blanks and then drill them out. I thought you could just by them sized like a jet but you cant.

  6. #6
    Senior Member DYNO OPERATOR
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    Yes you can drill them out to match them. No confusion there, but if you radius the entry hole it will act much leaner or bigger if everything is not identical. The depth of the hole will effect the bleed as well. some bleeds have to be drilled all the way through about 1/4" total depth, and some only require about .030 depth to get to the other side of the bleed. No biggie either way that creates a minor change. You may look at to see of you are running less fuel pressure than on the dyno too. The lower fuel level can make a lean hole when the booster tips in off the idle / transition circuit. Go about 4 hundredths smaller on the bleed to test, that is about the same as a jet change on holleys and enough to tell if you have the bleed too large quickly. I have never done part throttle pulls on the dyno before but you may have had this condition you are fighting present on the dyno, but did not show itself and behaved like you said it behaves when you slap the throttle and runs fine. The power valve may be too low a number and causing a lean condition in the car that the dyno would have never measured or known because it was wide open flowing fuel the whole time the test was run, not part time like on the street........... Good luck hunting, if you have reason to believe that the bleed is drilled bigger than what you dyno'ed with than you are likely correct on the fix but driveability problems are common on dynoed engines that do not run at wot like they are tested.
    Bruce

  7. #7
    Senior Member RACING JUNKIE
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    Re: High speed air bleeds

    Quote Originally Posted by bryanm
    I just got my motor back in my car and running but now it has a little stumble from about 3500-4200 rpm when you ease into the throttle. If I just whack the throttle hard it revs clean. It did not do this on the dyno but I had my high speed air bleeds on my Demon carb drilled out to supposedly the same size that they were on the dyno. I am thinking that they got drilled out to big. What do you guys think, is that the cause?
    Is this a drag car or street car? Are you running racing fuel or pump gas?What kind of HP numbers on the dyno?

  8. #8
    It is a drag car and runs on race fuel. It is a sbf, it made 580hp and 540 ft/lbs torque. I am wondering if this will even show itself on a full blown pass. I will start with the air bleeds and go from there.

  9. #9
    Senior Member RACING JUNKIE
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    I really don't think the air bleed is the problem. Those demon carbs seem to never be clean from idle to wot. They usually have a bad spot somewhere. I usually change plugs after I break in an engine. My advice would be examine and change plugs. And change carb.....If you still have the problem you can eliminate the carb as the source.

  10. #10
    I have already changed plugs. The carb worked fine on the motor last year with the old heads, it has never been an issue.


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